Colloid means a system in which disperse phases having a size of about several nm to several μm (e.g., colloidal particles) are dispersed in a dispersion medium. The colloidal particles in the colloid can be arranged regularly to form an orderly structure under special conditions, which is called colloidal crystals.
Similarly to usual crystals, the colloidal crystals Bragg-diffract electromagnetic waves according to the lattice spacing. The diffraction wavelength can be set to a visible light range by selecting production conditions (e.g., particle concentration, particle diameter, and refraction index of particles or medium). Therefore, application development to an optical element or the like including a photonic material has been actively studied both nationally and internationally. The present mainstream of a producing process of an optical material includes a multilayer thin film process and a lithography process. Both of the techniques can produce colloidal crystals having excellent periodic accuracy. However, the former provides only a one-dimensional periodic structure, and the latter provides only the one-dimensional periodic structure or a two-dimensional periodic structure.
There are three kinds of colloidal crystals.
The first type is the colloidal crystal in a hard sphere system on which only hard sphere repulsion works between particles. This colloidal crystallization depends only on entropy, and the particle concentration is the only one concrete parameter. This is similar to a phenomenon that macroscopic spheres are regularly arranged when they are stuffed into a limited space, and the volume fraction of the crystallized particles is about 0.5 (concentration=50% by volume) or more. At this time, crystallization occurs even if the particles are not in contact with each other.
The second type is the opal crystal, which is the generic name of a crystal structure packed with particles in contact with each other. The volume fraction depends on the crystal structure, and is, for example, about 0.68 for a body-centered cubic lattice, and 0.74 for a face-centered cubic lattice.
The third type is the charged colloidal crystal, which is formed by electrostatic interaction working between particles in a dispersion system of charged colloidal particles (charged colloidal system). The electrostatic interaction extends for a long distance, so that crystals can be formed even when the particle concentration is low (the interparticle distance is long), and the particle volume fraction is about 0.001.
There is a report that the colloidal particles having a uniform particle size precipitated, aggregated, and regularly arranged when they were used in a colloidal system with no special interaction between colloidal particles, and form closest-packed opal-type colloidal crystals (Patent Literature 1). However, for the opal-type colloidal crystals, only an aggregate composed of colloidal crystals having one lattice constant has been obtained, and there is no report on a eutectic colloidal crystal containing two or more kinds of colloidal crystals composed of monodispersed colloidal particles.
As an example of deposition of colloidal crystals from a multi-component colloid, Non-Patent Literature 1 reports the classification of gold fine particles and gold nano-rods, but the above-described eutectic colloidal crystal was not obtained therein. More specifically, this document reports that, from the mixed colloid of gold fine particles and gold nano-rods, only the gold fine particle alone formed colloidal crystals and gold nano-rods aggregated at the grain boundaries, which does not mean the formation of a eutectic colloidal crystal composed of the colloidal crystals of gold nano-rods and gold fine particles.
In a recently found phenomenon, plural kinds of opal-type colloidal crystals having different lattice constants coexisted in a dispersion medium (Non-Patent Literature 2). However, this colloidal crystal system is a state where plural kinds of opal-type colloidal crystals are suspended in a dispersion medium having the same specific gravity, but not mixed together at fixed positions to form an aggregate. Therefore, the direction of the optical axis of the colloidal crystals can be varied by Brownian movement, and thus the application to optical devices such as photonic materials is difficult.